1. Field of the Invention
The present invention relates to a piezoelectric filter including a plurality of juxtaposed dielectric ceramic resonators, and a method of adjusting its frequency bandwidth.
2. Related Background Art
There have been proposed various dielectric filters comprising a dielectric ceramic block including a plurality of juxtaposed resonators. In the dielectric ceramic block a plurality of through holes are provided in juxtaposed configuration. Each of the through holes is formed with an inner conductor on its inner wall. The dielectric ceramic block has an outer conductor provided on the outer surface portions thereof except an open circuit end surface portion at which one end of each through hole meets.
One example of such conventional dielectric filters is shown in FIG. 1 of the accompanying drawings. The illustrated filter A comprises a dielectric ceramic block B including two resonators C1 and C2 therein. An outer conductor D is provided on the outer surface portions of the block B except an open circuit end surface portion E and input/output terminal pads F are provided on one lateral surface portion of the block B provided with the outer conductor D and are electrically separated from the outer conductor by peripheral gaps. On the open circuit end surface portion E a transverse strip conductor G is provided for separating the resonators C1 and C2 from each other. The strip conductor G has both ends connected to the outer conductor D. This strip conductor G is operative to provide polarization on a high band side. The electromagnetic coupling between the resonators C1 and C2 can be regulated by modifying the configuration and/or width of the strip conductor G.
Recently, with the spread of digital communication system such as mobile phone it is required to use a dielectric filter having a wider frequency band. However, with the conventional filter mentioned above, the provision of the transverse strip conductor on the open-circuit end surface of the block is operative to provide polarization on a high band side, but the frequency bandwidth is reduced. Meanwhile, the use of dielectric filter having a narrow frequency band is required for some applications. However, the dielectric filter having the arrangement mentioned above can not be adjusted to have a desired frequency band in accordance with the requirement for an application.